Positive Crankcase Ventilation. The real worry is it gets stuck in the closed position not allowing your crank case to vent. It hooks up to the air intake which causes a vacuum to place on the system. This helps vent blow by in the crankcase and is then burned instead of vented. Installing a catch can will trap the oil that is coming out but allow the vapor to continue and be burned. Having one stuck closed I guess could rob you of a little HP in turn robbing you of a little MPG. But that part I'm not sure about.